1. Field of the Invention
The present invention relates to a fan motor drive circuit; more particularly, relates to a DC brushless fan motor drive circuit capable of detecting the operation of the fan motor.
2. Description of Related Art
FIG. 7 is a circuit diagram showing a conventional drive circuit for driving a DC brushless fan motor. The drive circuit comprises a Hall effect sensor 40 for detecting the polarity of a rotor formed of a permanent magnet in the fan motor, a first transistor 10 connected to a first magnetic coil L1, and a second transistor 20 connected to a second magnetic coil L2. The Hall effect sensor 40 has one input terminal connected to a fan motor power input Vin and the other input terminal connected to a grounding line. The first transistor 10 has a collector connected to the fan motor power input Vin through the first magnetic coil L1, an emitter connected to the grounding line and a base connected to an output terminal of the Hall effect sensor 40. The second transistor 20 has a collector connected to the fan motor power input Vin through the second magnetic coil L2, an emitter connected to the grounding line and a base connected to the collector of the first transistor.
In use of the drive circuit of FIG. 7, when the fan motor is powered on, the first transistor 10 is turned on and current is supplied to the first magnetic coil L1 thereby generating a magnetic field to rotate the rotor. Then, the Hall effect sensor 40 detects the change of magnetic polarity caused by the rotation of the rotor thereby generating a high level signal to turn off the first transistor 10 and turn on the second transistor 20 for supplying current to the second magnetic coil L2. Again, the magnetic coil L2 generates a magnetic field to rotate the rotor. Consequently, the first transistor 10 and the second transistor 20 are alternately turned on and turned off for supplying current to the first magnetic coil L1 and second magnetic coil L2 respectively thereby continuously rotating the rotor.
The conventional drive circuit as shown in FIG. 7 is only used for rotating the rotor of the fan motor. It is hard to know whether the operation of the fan motor is normal or not. However, a fan motor may be unstable or even out of order after running for a long time, which may affect other electronic elements cooperated with the fan motor in an environment where heat dissipation is strictly important. Therefore, it is desired to detect the operation of the fan motor from the drive circuit. One solution is to provide the drive circuit with a signal detection line connected to the base or collector of the first or second transistor 10,20. From such a signal detection line, a square wave signal, whose frequency is proportional to the rotating speed of the fan motor, is obtained and the operation of the fan motor can be known. However, the amplitude of this square wave signal is restricted by the fan motor power input Vin, and as a result, it can not be adjusted to meet the actual requirement. Moreover, when the fan motor is out of order and stops running, the detected signal presented on the signal detection line is unpredictable; more specifically, the signal detection line may present a high level signal or a low level signal depending on the position where the rotor stops. Therefore, there is a continued need for the above drive circuit to be improved.
The present invention provides an improved drive circuit for a DC brushless fan motor to mitigate and/or obviate the aforementioned problems.